This invention relates to a printing screen, sometimes referred to as a stencil or mask, for example for use in the screen printing of electronic substrates such as circuit boards with conductive tracks or the like. Whilst one specific intended use of the invention is described, it will be appreciated that the invention may be used in a range of other applications.
In a known screen printing arrangement, a thin, stiff foil, for example of stainless steel form, has a series of openings formed or cut therein to form the apertures through which the fluid to be printed onto a substrate is delivered, in use. The fluid may comprise, for example, a solder paste. The manner in which the apertures are formed in the foil is not of relevance to the invention.
It is important to ensure that, in use, the foil is held under tension. In some arrangements this is achieved by permanently mounting the foil in a suitable tensioning frame. Whilst this achieves the desired effect of ensuring that the foil is under tension, the cost of providing each foil with its own tensioning frame is excessive. Furthermore, the space required to store each foil and associated tensioning frame is large. Rather than provide each foil with its own tensioning frame, arrangements are known in which the foil can be temporarily fitted into a tensioning frame when required, and can be removed therefrom after use. In order to allow the mounting or fitting of the foil to a tensioning frame, it is known to provide each foil with a sub-frame, secured to the periphery of the foil, the components of the sub-frame being designed in such a manner that they can cooperate with the tensioning frame so as to permit tensioning of the foil.
Techniques for securing a sub-frame to a foil are described in, for example, WO03093012. In that arrangement, the foil is formed along its edges with castellations which are folded back, out of the plane of the foil, and are located within, grooves formed in the sub-frame components.
Whilst the arrangements described in WO031093012 may operate successfully, an alternative, simple technique for attaching a screen printing foil to a sub-frame is desired.